Apple's servers determine which iPhones are unlocked or not.
When you pay your Carrier for an unlock, they usually pass your IMEI number to Apple who then verifies it and then adds it to the whitelist.
IMEI unlock sites like those you mentioned need to be taken with a pinch of salt, as from what I've heard, they normally have insiders within the Carrier company who can "smuggle" your IMEI into their list of codes to be sent to Apple.
Or they hack Apple servers, who knows.
Buyers' beware in any case.